1. Field of the Invention
The present invention relates generally to a magnetron, and microwave oven and high frequency heating apparatus each equipped with the magnetron, and more particularly, to upper and lower pole pieces of a magnetron that carry magnetic flux generated by a permanent magnet in a magnetron into an activating space.
2. Description of the Related Art
As illustrated in FIG. 1, in a magnetron, a cathode including a filament 101 that emits thermions is disposed at the axial center of the magnetron, an anode including a plurality of vanes 102 that constitute resonance circuits and an anode cylinder 103 is provided outside the cathode, and an activating space 104, through which thermions emitted from the cathode move, is formed between the anode and the cathode. To cause the thermions to assume a certain type of movement, the rectilinear movement of the thermions is induced by an electric field caused by a potential difference generated between the cathode and the anode by the application of external electric power, and the rotational movement of the thermions is induced by a magnetic field applied to the activating space by upper and lower permanent magnets 105a and 105b. To carry magnetic flux generated by the two permanent magnets 105a and 105b into the activating space 104 (for ease of description, the rotation of a magnet in the direction from the north pole to the south pole thereof is ignored), upper and lower pole pieces 106a and 106b are provided between the upper permanent magnet 105a and the anode and between the lower permanent magnet 105a and the anode, respectively With the above-described construction, the thermions reach the anode while traveling spirally by electromagnetic force. At this time, rotational electron poles are generated around the cathode by the thermions and an induced current is generated in the resonance circuit of the anode, so that oscillations are incited and maintained. The magnetron is widely used in home appliances, such as microwave ovens, and is used in industrial applications, such as high frequency heating apparatuses, particle accelerators and radar.
Two permanent magnets are provided above and below the anode and function to render the movement of thermions uniform by forming uniform and symmetrical magnetic flux density in the activating space, thus suppressing the generation of unwanted noise. However, the provision of the two permanent magnets 105a and 105b increases the height, weight and volume of an overall magnetron. Additionally, the provision of the two permanent magnets 105a and 105b increases the manufacturing cost of the magnetron by increasing the number of assembly steps.
In order to solve the above-described and/or other problems, a configuration was proposed in which a single permanent magnet is disposed above the anode. As illustrated in the graph of FIG. 2, this type of configuration causes the movement of thermions to be non-uniform due to the non-uniform magnetic flux density thereof, so that a large mount of unwanted noise is generated, thus reducing oscillation efficiency. Accordingly, this type of configuration is employed only in small capacity magnetrons. In the graph of FIG. 2, the X-axis represents a distance ranging from the point of the upper pole piece to a certain point in a direction from the upper pole piece to the lower pole piece in millimeters, with a value “0” allocated to the point of the upper pole piece, and the Y-axis represents a magnetic flux density at the certain point in Teslas (Ts). Alternatively, to overcome the above drawback, there have been attempts to render magnetic flux density uniform by causing the tapered angles or center hole sizes of upper and lower pole pieces to be different, as disclosed in Japanese Pat. Unexamined Pub. No. Hei 5-41173. However, in accordance with these attempts, parts perpendicular to the axial center of an anode are maintained at the ends of the tapered surfaces of upper and lower pole pieces, so that the entire magnetic flux is dispersed, thus reducing the oscillation efficiency of a magnetron compared with the magnetic flux capacity of the magnet.